Roof geometry
Firring strip taper
- where
- rise (mm)
- horizontal length (mm)
- fall denominator (1:n)
- high-end thickness (mm)
- low-end thickness (mm)
- taper angle (°)
Explain this equation
Works out the thicknesses of a tapered firring strip that creates a set fall on a flat roof.
L is the horizontal length of the strip in millimetres. n is the fall denominator, so 1:40 means 1 mm of drop for every 40 mm along. r is the rise over the length. t is the thickness at the low end, and T at the high end. θ is the taper angle.
A 1:n fall changes height by L/n over the length, so the high end is the low end plus that rise. The angle is arctan(1/n), the same slope expressed in degrees.
A 4000 mm strip at 1:40 rises 100 mm, so a 20 mm low end becomes 120 mm. The taper is about 1.4°.
Watch out: the fall must come from the roof design. Allow for deflection, which can reduce the effective fall.
Calculate the geometric high end and taper for a specified fall.
Enter your values to see the result.
Good to know
The designer specifies the fall, supports and roof build-up. This does not account for deflection or choose a compliant design fall.
Sources
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.